Various techniques have been known in the prior art for erasing erroneously printed characters. These techniques include both lift off systems and cover up systems which have been commercialized for impact printers such as typewriters. These systems require an accurately registered overstrike of the character printed in error with a correction ribbon rather than the printing ribbon in place. However, in non-impact printing systems, no character shaped element is present so that conventional impact printing methods and apparatus cannot be used. Some prior art techniques were designed for correction which do not require accurate alignment of a character shaped element and thus were operable in principle to erase printing produced by non-impact printing techniques. One such system which utilizes impact correction techniques is shown in U.S. Pat. Nos. 3,862,679 and 3,866,736. This apparatus provides for a special key carrying a universal obliterating type font which is controlled in conjunction with a cover-up correction ribbon to produce a cover-up area large enough to cover any character within the type font being used. This correcting technique has not been well accepted for impact printing correction since the large cover-up area may be very noticeable.
In addition, this method would not be expected to produce good operation in a non-impact printing system due to the difference in printing characteristic between the plain paper and the large area of cover-up material. Other correction systems have utilized such techniques as lasers, heat sources, liquid cover-up or aerosols to produce a correction operation, and consequently required more complex methods and apparatus to perform a correction operation.